The dissertation is about the informal economy, which is broadly defined as the reverse of the formal economy. In particular, an in-depth explorative study of three Belgian employment markets (seasonal work, street trade and sex work) is conducted by means of a collective case study. In each case study, the presence of informal economic activities, their relations with formal and criminal activities and the motivations of workers to participate in the informal economy are studied. We furthermore identify the policy stipulations regarding seasonal work, street selling and prostitution, how this policy is perceived by different stakeholders and how it has an influence on informality. On the basis of our empirical results, we tentatively propose an alternative conceptualization of the relation between the formal, informal and criminal economy. The dissertation furthermore contributes to the regulation literature, by applying responsive regulation theory as developed by Ayres and Braithwaite (1992) to the empirical results in order to explore its usefulness for regulating the informal economy.